Carbiding

ABSTRACT

A METHOD OF APPLYING A CARBIDE COATING TO A SURFACE OF AN ARTICLE HAVING 0.5% OR MORE CARBON WHICH INCLUDES IMBEDDING THE ARTICLE IN A PACKED BED WHICH HAS AN UPPER PORTION AT A LOWER TEMPERATURE AND IN COMMUNICATION WITH THE ATMOSPHERE, THE CENDENSIBLE VOLATILES MOVING FROM THE HIGH TEMPERATURE LOWER PORTION CONDENSE IN THE UPPER PORTION AND ARE PREVENTED FROM BEING MOVED INTO THE ATMOSPHERE. THE INGREDIENTS OF THE PACK INCLUDE AN ANTIDUSTING AGENT, A METAL SOURCE COMPONENT, HALIDE OF THE METAL SOURCE COMPONENT, AND BARIUM CHLORIDE.

United States Patent O 3,579,373 CARBIDING Vernon J. Pingel, 812 W.Grove St., Arlington Heights, Ill. 60005 Filed Oct. 18, 1968, Ser. No.768,836 Int. Cl. C23c 11/08 U.S. Cl. 117-106 4 Claims ABSTRACT F THEDISCLOSURE A method of applying a carbide coating to a surface of anarticle having 0.5 or more carbon which includes imbedding the articlein a packed bed which has an upper portion at a lower temperature and incommunication with the atmosphere, the condensible volatiles moving fromthe high temperature lower portion condense in the upper portion and areprevented from being moved into the atmosphere. The ingredients of thepack include an antifusing agent, a metal source component, halide ofthe me-tal source component, and barium chloride.

BACKGROUND OF THE INVENTION Altering the surface of metal such as steeland nonmetals such as carbon or graphite articles to` provide anextremely high degree of hardness and abrasion resistance is of greatcommercial significance. It has been suggested that a layer of chromiumbe deposited on metals by methods which are commonly known as vapordeposition methods. These methods usually involve depositing a layer ofchromium on the surface of a metal article while residing in an inertatmosphere, reducing atmosphere, vacuum, or in a molten bath. Thesemethods are generally cumbersome. They are not entirely suitable for usein foundry-like conditions which require extreme simplicity inprocedures. These methods lead -to the deposit of chromium which alloyedwith the iron base almost immediately. Also, the materials used in themethods heretofore suggested were often difficult to handle, and lossesof these materials in the course of opera-tion tended to be expensive.It is an object of the present invention to provide a method fordepositing an extremely hard carbide layer on the surface of articlesusing a reusable powder bed which can be operated in simple equipmentwhile it is exposed or vented to the room atmosphere. It is a furtherobject of this invention to provide a method for depositing a carbidelayer on carbon articles, e.g. graphite, as well as on articles whichcontain relatively small amounts of carbon, e.g. 0.5% and above. It isan important object of this invention to provide a method for producingcarbide coatings which method involves virtually no loss of ingredients.

SUMMARY `OF THE INVENTION This invention relates to a method ofdepositing carbide coatings on articles containing 0.5% or more carbonat the surface thereof. The method involves the use of a powdered bedconfined in equipment having a first and second region. The article isimbedded in the bed in the first region and this region of the bed isheated to a ternperature of insipient fusion of the bed. A second regionof the equipment is located between the heated rst portion and theatmosphere even though the bed is in direct pneumatic communication withthe atmosphere through the second region. The second region of theequipment is maintained at a temperature below the temperature at whichcondensible volatiles moving from the heated region are condensed. Theonly pneumatic communication between the bed and the room atmosphere isthrough an orifice above the second region, a suiiicient height toprevent passage of condensible volatiles into the atmosphere,

3,579,373 Patented May 18, 1971 "Ice and to maintain protective heavyvolatile condensibles between the room atmosphere and the heated bedduring the process. The rate and quality of deposition on the articlecan be altered by application of an external electrical potential to thearticle.

DESIGNATION OF THE FIGURES FIG. 1 is a perspective schematic viewshowing a tube in place in an electric furnace;

FIG. 2 is an enlarged schematic cross-sectional view taken approximatelyalong the lines 2 2 of FIG. l;

FIG. 3 is a perspective view of a cylindrical article which wascarburized and then sawed off from the remaining portion of thecylindrical article prior to treatment in accordance with thisinvention;

FIG. 4 is an end view of the article shown in FIG. 3 as viewed towardsan end which has been carburized prior to treatment in accordance withthis invention; and

FIG. 5 is an end view of the article shown in FIG. 3 as viewed towardsthe end exposed by sawing the carburized article through prior totreatment in accordance with this invention.

DESCRIPTION `OF PREFERRED EMBODIMENTS Although the following `disclosureoifered for public dissemination, in return for the grant of a patent,is detailed to ensure adequacy and aid understanding, this is notintended to prejudice -that purpose of a patent which Iis to cover eachnew inventive concept therein no matter how others may later disguise itby variations in form or additions or further improvements. The claimsat the end hereof are intended as the chief aid toward this purpose; asit is these that meet the requirement of pointout out the parts,improvements, or combinations in which the inventive concepts are found.

In accordance with this invention articles 10 which contain 0.5 or morecarbon are imbedded in a bed 11 of powder, the makeup of which will bedescribed in greater detail herein. The powder bed is contained in asuitable vessel, or tube 12 (e.g. one made of Inconel, ceramic, or othersuitable material). The bed-containing tube 12 is placed in a furnace 13in such a manner that the tube may be considered as having two regions,namely the lower portion containing the bed that is being heated, andthe upper portion containing the bed that resides outside of the furnaceand is thus maintained at temperatures far below the temperaturesencountered within the furnace. Thus the lirst portion of the bed, whichmay be defined approximately as that portion between insulated top 14and insulated bottom 15, is subjected to the heating action of heatingelements 20 in furnace 13. This portion of the bed is heated to atemperature of insipient fusion. The meaning of this term will beexplained more fully hereinafter. After the operating temperature isreached in the heated portion of the bed certain condensible volatilesare formed and ow through the voids in the powdered bed. The condensiblevolatiles move upwardly in the illustrated embodiment to the secondregion in which the tube and the bed are not heated. In this region,particularly in the approximate region marked A in FIG. 2, thecondensible volatiles are condensed. The bed in the region marked Abecomes a readily friable, integral mass. In a preferred embodiment theportion of the bed which is maintained at condensing temperatures, i.e.in the second region of the equipment is sufficiently large to assurecomplete condensation in the bed of the condensible volatiles formed inthe heated portion of the bed. Naturally as the heating period isprolonged a greater and `greater quantity of condensibles are collectedin the region designated A in FIG. 2 and in many instances thequantities of material become large enough to virtually plug the bed,i.e. sever the pneumatic communication between the cooled upper andheated lower portions of the bed. f [n a less preferred embodiment theibed does not extend into the second, cooled region, and condensationtakes place on the walls of the vessel in the stagnant atmosphere ofthat region of that vessel.

Even if a plug does not form in the cooled region of the bed, the heavygases or volatiles can rise a limited extent in the cool region, thusgiving adequate protection to the atmosphere in the hot zone. Inaccordance with this invention it is essential that the direct pneumaticcommunication or orifice between the confines of the vessel and the roomatmosphere be at a sufficiently high vertical distance above the heatedportion of the bed that the apparent uppermost level of volatiles formeddoes not reach the orifice of the vessel at any time during the process.

BED MAKEUP The bed employed in accordance with this invention is made upof finely divided, preferably powdered, ingredients consisting of fourcomponents. The first component is an anti-fusing component and isselected from materials which retard or inhibit sintering or fusing ofthe remaining components. The first component is selected from materialswhich do not melt or become sticky at operating temperatures. Examplesof classes of materials from which the first component may be selectedare alundum, or other aluminum oxide materials, crushed rebrick,ceramics, etc. The first component is present in an amount between and70% inclusive, preferably 5%-50%.

The second component is a metal-source component and is selected fromthe group high carbon or low carbon ferrochromium, chromium,ferro-molybdenum, molybdenum, ferrotitanium, titanium,ferrotantalum-columbium, ferro-columbium, ferro-tungsten, and tungsten.The metal source component is used in an amount between 5%-80% inclusiveof the bed preferably between 5% and 50% inclusive of the bed by weight.

The third component is a metal halide in which the metal is one of themetals present in the metal source component, for example, chromiumchloride (CrCla). The metal halide component is used in an amountbetween about 5% and about 70% by weight of the bed preferably between7% and about 30% of the weight of the bed.

The fourth and also essential component is barium chloride. Thiscomponent is used in an amount between 7% and 80% by weight of the bedmore preferably between about 10% and 80% of the weight of the bed.

I believe that the' metal-source component contributes substantiallyalso to the heat conductivity of the bed, and therefore I prefer to useratios of ingredients which have a high metal-source component level. Ialso prefer to use a minimum amount of the first, anti-fusing, componentconsistent with safe operation of the bed. I believe that such lowlevels of the anti-fusing component permits maximum involvement ofcomponents actually involved in the generation of the coating. I havefound that ferrochromium silicon, which is generally recognized by theart to be non-equivalent to ferrochromium cannot be used in the methodof this invention. Beds in which ferrochromium silicon is used have anextremely short useful life after which they become inactive withrespect to forming the carbide coating. Also, it is essential thathalides of the metals involved be present. The substitution of chromiumoxide (CrZOS) for chromium chloride does not result in a useful bed.

However, some of the barium chloride can be replaced by melting pointdepression agents, such as sodium chloride or potassium chloride, (eg.10% to 50% by weight of the barium chloride) for the purpose of loweringthe fusion temperature of the salt. However, it is es sential that thebarium chloride be present in amounts as stated above. Elimination ofthe barium chloride from the ingredients of the bed results in a mixturewhich is not useful in accordance with this invention.

In this specification, and in the definition of the method of thisinvention the term temperature of insipient fusion is used. This termindicates not a single temperature but a range of temperatures which isabove the melting or sinterng temperatures of some of the ingredients ofthe bed and yet is below the temperature at which the bed will fuse orsinter and lose its identity as a powder or readily friable mass. Thusin a bed consisting of 33.5 alundum, 33.5% ferrochrome, 16.5% bariumchloride and 16.5% chromium chloride temperatures between 1700 and 2000are in the range of temperature of insipient fusion, particularlytemperatures in the range 1800 to 2000. The temperature of fusion of amix can be readily determined experimentally by a simple test whichconsists of placing a number of samples in a furnace and graduallyraising the temperature of the furnace while removing samplesperiodically. After cooling, the samples are subjected to light tappingand those samples which have not yet reached the temperature of fusionwill readily disintegrate and return to the powder condition. On theother hand, samples which have reached the fusion temperature solidifyinto an integral mass which can no longer be considered readily friable.While I do not want to be bound by any theories it is my understandingthat the fusion of the bed into an integral mass as a result ofexceeding the temperature of fusion of the bed involves the results ofmelting or sintering, or both, of the ingredients.

While both high and low carbon ferrochromium will work in accordancewith this invention, high carbon ferrochromium, e.g. ferrochromiumcontaining 65 70% chromium and 4%-9% carbon, the balance being iron, ispreferred. However, it appears that low carbon ferrochromium generallygives a higher shine coating than the high carbon ferrochromium.

The source-metal halide can be added to the other ingredients of the bedas an aqueous solution, but the resulting wet mixture must be driedprior to use as a coating bed or severe metal corrosion may result.However, the application of the chromium chloride to the remainingingredients by way of a concentrated chromium chloride aqueous solutionfollowed by drying of the `wet ingredients produces a highly reactivebed mixture. However, the practical difficulties encountered due to thecorrosiveness of the aqueous chromium chloride are generally notsuicient to justify this method because of the increase in reactivity.Consequently, simple admixing of the components in the form of drypowdered ingredients is generally the most practical and suitable infoundry-like operations.

Molybdeum carbide can be applied as a coating if ferromolybdeum, ormolybdeum is employed as metal source with molybdeum chloride. It ispreferred that the metal source component be 55%-75% Mo, 0.1 to 2.57 C,balance being iron.

Tungsten carbide is applied if ferro-tungsten (eg. 65%- 82% tungsten,.25 to 2.50% carbon, balance iron), or tungsten is used in conjunctionwith tungsten chloride.

Tantalum carbide can be coated if pure tantalum is used; or tantalum andcolumbium carbides can be coated in accoradnce with this invention if ametal source such as ferro-tantalum-columbium (20% Ta, 40% Cb, .20% C,bal. Fe) or ferro-columbium (58% to 62% Cb, bal. Fe) is emploeyd, withtantalum chloride and/or columbium chloride.

Titanium carbide can be applied by use of ferrotitanium (e.g. 15%-20%titanium, l%-2% aluminum, 2%-3% silicon, 3%-8% carbon, balance iron) ortitanium in conjunction with titanium chloride.

While I do not want to be bound by any theories it is my belief that themetal-source component resupplies the metal chloride with the metal ionsthat are taken from volatile chloride as it reacts to form the carbidecoating. It is my understanding and belief that chromium chloride Vapor(CrClz) is present in the voids in the heated portion of a bedcontaining CrCl3 powder.

CARBON CONTENT OF ARTICLES I have found that the carbide coating can beincorporated on the surface of articles which contain 0.5% carbon ormore, or on articles which have a carbon coating thereon. Thus, steelssuch as 1020 steels (0.20% carbon) are not suitable for use inaccordance with this invention. However steels such as the 1060 and the52100 steels (0.60% and 1.00% carbon, respectively) are entirelysatisfactory for use in accordance with this invention.

Likewise, I have discovered that painting or printing ceramics or otherobjects, such as copper sheets, with a carbon-containing orgraphite-containing ink provides a surface which can be carbided inaccordance with this invention. One such ink is DAG (trademark ofAtchison Colloidal Graphite Company) which is a colloidal suspension ofgraphite in a suitable carrier such as kerosene, water, alcohol, etc. orin admixture with a greaselike material. Subjecting an article imprintedwith such a material to the process of this invention results in adeposition of a metal-like coating in the areas in which the printingoccurred. While I do not want to be bound by any theories it is myunderstanding and belief that the operating temperatures of the methodof this invention, being just below the melting point of copper, causesa carbide coating formed by reaction with the carbon imprintation oncopper to be diffused and fused into the copper underlayment. Thus, itwill be appreciated that not only is this method highly useful in thefine arts, but it is also of considerable use in the preparation ofetching, etc. The high chemical resistance of the carbide coating isutilized to permit selective dissolution of the uncoated copperunderlayment leaving the carbide coated regions substantiallyunaffected. Such etchings have unusually hard elevated surfaces.

On the other hand articles made of graphite can be' partially shieldedfrom the application of a coating in accordance with this invention byelectroplating a fine copper coating on the area to be protected andsubsequently treating the entire article in accordance with thisinvention. The result is that the carbide coating is not laid down overthe copper layer but is laid `down on the unprotected regions of thecarbon or graphite article. The examples presented hereinafter willfurther illustrate the importance of the proper carbon level in themethod of coating articles in accordance with this invention.

In the examples and throughout the specification the reference topercent are percent by weight, and parts is expressed in parts byweight. Also, all references to temperatures are expressed in degreesFahrenheit.

Example 1 A number of articles having the carbon contents set forth inTable I were placed in a bed 11 having the following makeup: alundum(33.5%), ferrochrome (33.5%), chromium chloride (CrCl3, 16.5%), bariumchloride (BaCl2, 16.5%). (The bed was 100%-6 mesh, 90%-20 mesh.) Thisbed 11 and articles 10 are placed in the loweremost portion of anInconel tube 12 which is closed at the lowermost end. Additional outermaterial having the same makeup as the rst portion of the bed is addedto fill the elongated lnconel tube to within a few inches of the openend 2S of tube 12. Tube 12 thus charged, is placed vertically in asuitable furnace 13,

which is constructed to heat only the lowermost portion of the chargedtube which contines the portion of -bed 11 in which articles 10 reside.The uppermost portion of the tube extends outside of the furnace and ismaintained at relatively low temperatures, due to the cooling effect ofthe room atmosphere. Furnace 13 is energized to heat tube 12 and thelower region of its contents as rapidly as possible to a temperature ofabout 1900 degrees F. This took approximately one hour. The tube and itscontents were maintained in the furnace at l900 F. for two additionalhours. At the end of the two hours the tube was removed from the furnaceand was permitted to air cool to room temperature. The tube was invertedand the extreme topmost portion of the bed poured readily from the tube.However, a plug was found approximately one-half way down the depth ofthe tube approximately in the region marked A in FIG. 2. This plugdisengaged itself from the tube when the tube was tapped lightly and theremaining portion of the contents poured readily from the tube. The plugwas readily friable and slight pressure returned the plug to arelatively powdery condition. The results of the demonstration aretabulated in Table I below.

TABLE I Carbon,

Article percent Result 0 00 Unacceptable as a coating,

metal appeared corroded,

20 surface extremely mottled, 4 Rockwell hardness, ranged C 20 anddownward. .60 Shiny, hard (C 72) suggestive of mottled character.Smooth, hard, very shiny. Smooth, hard, appears polished. Smooth, hard,shiny coating. Coating has white, lustrous appearance, depending oncondition of undercoat.

Armco ingot, iron Steel:

Cast iron Pure carbon (amorphous) Graphite Example 2 Articles fabricatedfrom a high carbon, high chrome steel -known as crucible Air-Kool steelwere imbedded in a series of beds having the same makeup as that setforth in Example 1 above and each bed was subjected to the conditionsset forth in Example 1 above except that different size beds andfurnaces were employed. At the end of the two hour uniform temperatureheat treatment period, the furnaces were shut off and the tubes werepermitted to remain in the furnaces while the furnaces cooled to roomtemperature. Upon removal of one of the tubes after the cooling periodwas completed it was found that the core of the articles had amartinsitic structure. In the tests in which a small furnace and a small(l1/2 inch diameter) tube Were employed the cores were very hard, andbrittle, and the coating was a hard and brittle coating. These materialsare heated to temperatures of 400 F. to 1000 F. depending on thecomposition of the steel and permitted to cool in air. This heattreating step resulted in a tough core with the hard case of chromiumcarbide remaining unaffected.

When a large bed is employed in a large furnace in the procedure of thisexample, so that the cooling period is a prolonged one, the result isthat the core is annealed, i.e. relatively soft and flexible. Yet thearticle has an extremely hard case. These articles permit flexing of thearticle without damaging of the piece.

In accordance with this invention the product of the process of thisinvention can be heat treated without the need of a protectiveatmosphere. In some instances the coating appears to turn dark but canbe cleaned off by light brushing, etc. However no signicant change inhardness of the case is found to occur as a result of the change 1nappearance.

Example 3 A number of drill rods fabricated from SAE 1090 steel (0.90%carbon) was coatedrby the process of this invention as outlined inExample l in a series of tests. The rod in a first test of the serieswas permitted to remain in the bed and the bed to remain in the furnaceuntil the entire mass had cooled to substantially room temperature. Theresult was that the core of the drill rod treated in this manner wasannealed or normalized, i.e. the core is soft and flexible. The rod hada hard, relatively brittle case. However, the rod can be easily bentwithout substantial damage to the case. In a second series of tests,rods coated in accordance with the procedure set forth above were heatedto a temperature of 1650 F. and were subjected to an oil quench. Theresult was that the cores were relatively brittle even though the caseshad substantially the same characteristics as the case obtained by theprevious heat treatment set forth in this example. Upon heating of theoil quenched articles to a temperature of 400 F. to 600 F. and aircooling the articles, the cores were converted to a condition offlexible toughness and the cases remained unaffected.

The examples set forth in numbered Examples 2 and 3 above are providedto illustrate that the articles produced in accordance with thisinvention do not require special atmospheric protection during thecoating step, nor during heat treatment of the coated articles for thepurpose of altering the characteristics of the core. The conventionalheat treatment procedures have virtually no effect on thecharacteristics of the case produced in accordance with this invention.

Example 4 An article fabricated from steel having a carbon content lessthan 0.6% and having the shape of a cylinder was carburized in aconventional manner to alter the surface characteristics of thecylinder. An end of the cylinder was then sawed off to sever that endportion from the remainder of the cylinder. The end portion 30 thus hadtwo ends, the carburized end having a high carbon content, and the cutend having a varying carbon content ranging from very high at the formerouter and inner edges to below 0.6% in the median areas of the cut, ex'posed end. The outer and inner surfaces of the cylinder have a highcarbon content. This article was treated in accordance with thisinvention under the procedure set forth in Example 1 above. Theresulting coated article was found to have a uniformly high shinypolished appearance at the outer and inner circumferences of thecylinder and at the carburized end of the cylinder as viewed in FIG. 4.However the cut end was not uniformly coated. As viewed in FIG. 5, theregion 31 adjacent the outer circumference of the cut end and the region32 adjacent the inner circumference of the cut end had a highly lustroushard uniform coating and the median region 33 which was originally theinterior of the carburized article had virtually no coating depositedthereon as a result of the procedure of this example also illustratedthe drop in ample is to illustrate the use of this invention on articlesfabricated from relatively low carbon steels and which, withoutpre-carburization, are not coated in accordance with this invention.However they can be coated in accordance with this invention afterhaving been surface treated by carburizing or other conventionalprocesses to raise the carbon level at the surface thereof. Incidentallythe procedure of this example also illustrated the drop in carboncontent of the cut cross section of the cylinder with increasing depth,as reflected by the gradient from highly lustrous uniform coating in theregion of shallow depths gradually diminishing in uniformity and qualityas the depths increased until the innermost region 33 showed virtuallyno coating and a rather corroded general appearance.

Example S A sheet of copper was imprinted with a colloidal carbonsuspension in kersosene (DAG), and the carbon imprint was permitted todry. The sheet of copper was treated in accordance with this inventionunder the procedures outlined in Example 1 above after which the coppersheet was found to have a shiny white imprint conforming exactly to thecarbon imprint. The shiny white imprint was found to be extremely hardcarbide material which resisted etching action of conventional copperetching solutions.

Example 6 The procedure of Example 5 was repeated except that the carbonimprint was made on a ceramic article. The result was that the ceramicarticle had incorporated therein a `white shiny hard metallic appearingimprint conforming exactly to the carbon imprint previously impartedthereto.

Example 7 A graphite crucible was treated in accordance with theprocedures set forth in Example l above, except that a four hour periodat operating temperature was employed. The result was thatt he cruciblehad a highly dense, metallic-appearing layer at the entire surfacethereof this layer being highly resistant to the action of chemicals.

Example 8 Conventional burnishing balls were treated in accordance withthe procedure set forth in Example l. However, in order to avoid anypossibility of contact between burnishing balls while they were imbeddedin the bed of this invention, the individual balls were wetted with oiland then rolled in the powder used to make up the bed. The individualballs were thus covered with a layer of the bed powder whichautomatically separated the balls from one another when they were placedin the bed. After treatment in accordance with the procedure set forthin Example 1 it was found that the coating imparted thereto was anextremely hard, shiny, coating having a polished lustrous appearance andhaving a thickness of .0005". Thus the procedure of this invention is anexcellent convenient technique for modifying the diameter of articlessuch as burnishing balls in a controlled fashion to achieve smalluniform increase in the dimensions thereof.

Example 9 Ball-bearings made of 52100 steel were treated in accordancewith the procedure set forth in numbered Example 8. The resulting coatedball-bearings were found to have an extremely hard surface having asmooth polished metal appearance. Ball-bearings coated in accordancewith this procedure were placed next to otherwise identical untreatedball-bearings on a smooth inclined plane. Both sets of balls werereleased simultaneously and due to the improved surface characteristicsthe balls treated in accordance with this invention reached the bottomof the inclined plane ahead of the untreated ballbearings.

Example 10 Steel rods of 1090 steel (0.90% carbon) were irnbedded in abed in a tube as described in Example 1 above, except that the rods weresufficiently elongated to pass through both portions, i.e. through theheated and uuheated portions of the bed, and to extend into theatmosphere above the bed. A sensitive device for detection of electricalpotential was connected to the rods and to the Inconel tube confiningthe bed. At room temperature no electrical potential was observed. Asthe heating progressed to about 400 F. denite polarity was observed, thetube being negative and the parts imbedded in the bed being positive.This polarity prevailed until the temperature of about 1700 F. wasencountered and within a very small temperature range at 1700 F. thepolarity -was observed to reverse. Thus at temperatures above 1700 F.the part within the bed showed a negative polarity and the Inconel tubeconfining the bed showed a positive polarity.

In a separate experiment the procedures outlined immediately above inthis numbered example was repeated except that two rods were imbedded inthe bed and were connected to the positive and negative terminals,respectively, of a 6-volt storage battery. After two hours at operatingtemperatures, and cooling to room temperature, the rod attached to thenegative electrode was found to be vastly superior with respect tocoating quality to the rod attached to the positive electrode.

Example 11 The procedure of Example 1 Was repeated except that the bedwas made up of the following ingredients: alundum (16.5%), ferrochrome(16.5% chromium chloride (CrCl3) (8.2%), barium chloride (58.8%). Theresults obtained in this example were substantially the same as theresults obtained in Example 1 with respect to a coating quality. Howeverit was observed that there was a more severe temperature lag on theinterior of the bed presumably due to the fact that there was asubstantially lower level of metal-source ingredients present in the bedof this numbered example.

Example 12 The procedure of Example l was again repeated except that thebed 'was made up of the following ingredients: Alundum (16.5%ferrochrome (66.6%), chromium chloride (CrCl3) (8.4%), barium chloride(8.5%). The results obtained in this numbered example were substantiallythe same as those obtained in Example 1 and reported in Table I therein.However, it was observed that the temperature lag of this numberedexample was considerably less than the temperature lag in the bed ofExample 1. I believe this is due to the presence of the extremely highlevel of metal-source component in the bed of this numbered example.

Example 13 The procedure of Example 1 was again repeated using theingredients listed in Example 1 except that 10% sodium chloride based onthe Weight of the barium chloride was added to the ingredients. In smallpreliminary tests it was found that the addition of the sodium chlorideto the barium chloride lowered the temperature of insipient fusion withthe result that the contact temperatures above 1500l F. could beemployed. Thus in this numbered example the furnace was set to provide amaximum of bed temperature of approximately l575 F. and the bed wasmaintained at that maximum temperature for approximately 2 hours. Theresults obtained in accordance with this numbered example weresubstantially the same as the results reported for Example l above.

Example 14 The procedure of Example l was again repeated using theingredients as set forth in Example l except that the maximum bedtemperature 'was limited to 1700 F. The bed was maintained at maximumtemperature for approximately 2 hours. After the bed and its contentshad cooled to room temperature the tube was emptied. It was observedthat substantially no plug had formed within the bed and that the degreeof coating on all of the articles treated in accordance with thisexample was entirely inadequate. In a separate experiment this procedurewas repeated except that a maximum temperature of 2150 F. was utilizedas the maximum bed temperature. It was observed however that the heatedportion of the bed fused into an integral mass and was not suitable forimmediate reuse in accordance with this invention.

I have observed that the maximum depth of the layer formed by the methodof this invention is approximately .0005 inch in thickness. Thus themaintaining of the article in the bed at operating temperature beyond 2hours does not provide a substantial increase in the thickness of thecoating. However the maintaining of the bed for times substantiallygreater than 2 hours does lead to a 10 change in the density of thecoating. Thus when extremely dense surfaces are required, e.g. surfaceson a crucible it is preferred that longer periods of time be utilized,e.g. 4 hours.

However, on surfaces intended to carry lubricants it is preferred thatshorter periods of time, e.g. less than 2 hours be employed, and thatsteels such as 1060 steel (0.6% carbon) be used. These steels areusually not uniform with respect to carbon distribution at the surface,the surface exhibiting both high carbon areas and low carbon areas. Theuse of such steels in accordance with this invention leads to arelatively uneven surface, microscopically speaking, there being a highdegree of coating at the high carbon areas and a lower degree of coatingat the lower carbon areas. Nonetheless, the overall appearance is thatof a relatively polished metal. It is my belief that the microscopicunevenness is highly advantageous insofar as the surface is permitted tocarry greater quantities of lubricant. Thus, in the coating of articlessuch as sealing rings, e.g. piston rings, I prefer to use ringsfabricated from 1060 steel (0.6% carbon) and coat in accordance with theinvention under conditions which provide a slight microscopic mottlingeffect. Using the mix described in numbered Example 1 above, thepreferred contact time at operating temperatures is about 3 hours forthe coating of articles such as piston rings.

Another advantage which results from the fact that the coating appliedin accordance with this invention can be an extremely uniform coating isthe fact that these coatings lead to virtually no change in the surfaceconguration of the article being coated. Thus irregularly shapedarticles such as files, saw blades, drills, taps, surgical knives andthe like, can be coated in accordance with this invention with theresult that an extremely hard, abrasion resistant layer is formedthereon without significant alteration in the surface characteristics ofthe article. No edge buildup is observed as is common in chromiumplating.

Balls and seats (1 diameter ball- 52100 steel-1.00% carbon) designed foruse in an oil Iwell pump were coated in accordance with this invention.They had a useful life more than three times longer than the identicaluncoated components when subjected to actual use in -lield conditions.

Articles coated to increase wear usually need no special treatment tore-distribute carbon to harden and back-up the micro-region immediatelybelow the coating of this invention. Thus ejection pins designed for usein a plastic extruder needed no further treatment. However, I wouldprefer to increase carbon back-up of articles coated in accordance withthis invention, if the article is to be used under high impactconditions. Thus I prefer to hold dental drills coated in accordancewith this invention at 1600 F. for one hour as a post-treatment topermit migration of carbon from the interior of the article to theregion under the coating. This hardness backs-up the coating making iteven more suitable for high impact use.

While a special atmosphere is not necessary when heat treating articlescoated in accordance with this invention, I prefer to protect articleswhich are treated at extremely high temperature, such as high speed toolsteel (i.e. tungsten type) articles which (after coating) are hardenedby quenching from about 2000 F.

As used herein, the term halide of chromium is intended to include,among the other halides, chromic chloride and chromous chloride andmixtures thereof. I have found that when hydrated chromic chloride isused as the halide bed ingredient, chromous chloride is present insubstantial quantities in the bed after the first hearing cycle. Iprefer, however, CrCl3-xH2O as a bed ingredient, because it is readilyavailable commercially, and has proven eminently satisfactory as aningredient.

I claim: 1. The method of applying a carbide coating to a 1 1 surface ofan article having 0.5% or more carbon, including the steps:

(1) Imbedding said article in a powder bed comprising the mixture (a) to70% by weight of a first powdered ingredient selected from the groupalundum, clay, firebrick, ceramics, said first ingredient being selectedfrom materials which do not become tacky at temperatures encountered inthis method,

(b) 5% to 90% of a second powdered ingredient selected from the groupferrochrome, chromium, ferromolybdenum, molybdenum, ferrotitanium,titanium, ferrotantalum-columbium, ferrocolumbian, ferrotungsten,tungsten, and mixtures thereof,

(c) 5% to 70% of a halide salt of the nonferrous metal selected as thethird ingredient, and

(d) 7% to 80% barium chloride,

said powder bed being in pneumatic communication with the atmosphere andbeing in the first portion of a vessel having a first and secondpoltion, the first portion being in direct pneumatic communication withthe second portion, the second portion separating the first portion fromthe atmosphere;

(2) heating the portion of the vessel and the bed to a temperature ofinsipient fusion of the bed;

(3) maintaining the second portion at a lower temperature at whichcondensible volatiles moving from the heated first portion arecondensed, and are thereby prevented from being moved into theatmosphere; and

(4) maintaining the article in the heated first portion for a period oftime suiiicient to deposit the required thickness of carbide depositsthereon.

2. A method of applying a chromium carbide coating to a surface of anarticle having 0.5 or more carbon, including the steps:

(1) imbedding said article in a first portion of a powder bed comprisingthe mixture (a) 5% to 80% of a first powdered ingredient selected fromthe group alundum, clay, firebrick, ceramics, said first ingredientbeing selected from materials which do not become tacky at temperaturesencountered in this method,

12 (b) 5% to 70% of a second powdered ingredient selected from the groupferrochrome, and chromium, and mixtures thereof, (c) 5% to 70% chromiumchloride, and (d) 7% to 80% barium chloride,

said powder bed being in pneumatic communication with the atmosphere andhaving a first portion and a second portion, the first portion being indirect pneumatic communication with the second portion, the secondportion residing between the first portion and the atmosphere;

(2) heating the rst portion of the bed to a temperature of inspientfusion of the bed;

(3) maintaining the second portion at a lower ternperature at whichcondensible volatiles moving from the heated first portion are condensedtherein, and are thereby prevented from being moved into the atmosphere;and

(4) maintaining the article in `the heated first portion for asufficient period of time for a deposit of the desired thickness to formthereon.

3. The method of claim 2 in which the bed comprises the mixtureincluding: alundum in an amount between 5% and 50% inclusive; a memberselected from the group ferrochromium and chromium, in an amount between5% and inclusive; chromium chloride (CrCl3), in an amount between 7% and30% inclusive, and barium chloride in an amount between 10% and 80%inclusive, all amounts being expressed in precent by weight of theweight of the bed.

4. The method of claim 3 in which the first portion of the bed is heatedto a temperature between 1800" and 2000 F. inclusive.

References Cited UNITED STATES PATENTS 2,874,070 2/1959 Galmiche1l7-107.2 2,885,301 5/1959 Samuel 117-107.2X 2,978,358 4/1961 Campbell117-106X ALFRED L. LEAVI'IT, Primary Examiner W. E. BALL, AssistantExaminer U.S. Cl. X.R. 117-107.2

